1. Field of the Invention
The present invention relates to a substrate cleaning method and a substrate cleaning apparatus for removing particles adhering to a substrate to be processed by immersing the substrate to be processed into a cleaning liquid and irradiating ultrasonic waves to the cleaning liquid. In particular, the present invention relates to a substrate cleaning method and a substrate cleaning apparatus capable of restraining a variation in particle removal efficiency between lots.
In addition, the present invention relates to: a program that accomplishes the substrate cleaning method for removing particles adhering to a substrate to be processed by immersing the substrate to be processed into a cleaning liquid and irradiating ultrasonic waves to the cleaning liquid, the substrate cleaning method being capable of restraining a variation in particle removal efficiency between lots; and a program storage medium storing the program.
2. Description of Related Art
There has been known, for example, from JP10-109072A, a method of cleaning substrates to be processed by immersing the substrates to be processed into a cleaning liquid and generating ultrasonic waves in the cleaning liquid, i.e., a so-called ultrasonic cleaning method (including a megasonic process). In the ultrasonic cleaning method, in particular, in a case where a chemical liquid is used as a cleaning liquid, the cleaning liquid overflowing from a cleaning tank is often circulated and supplied again to the cleaning tank so as to be reused.
When substrates are consecutively cleaned by the ultrasonic cleaning method, a removal efficiency of particles sometimes largely varies between lots. Particularly when a cleaning liquid is circulated and supplied again, a variation in the removal efficiency tends to increase.
The present inventors have conducted extensive studies on this problem, and found that, when a plurality of lots are continuously subjected to the ultrasonic cleaning process, a dissolved gas concentration of a gas dissolved in a cleaning liquid will change over time because of irradiation of ultrasonic waves and/or dissolution of an atmosphere. It was also found that, particularly when the cleaning liquid is circulated and supplied, the dissolved gas concentration largely changes. FIG. 8 shows a change in the dissolved gas concentration over time, when four lots were sequentially cleaned with the use of a cleaning liquid that was circulated and supplied again. In this example, although the first lot and the fourth lot were cleaned under the same process condition (240 W, 5 minutes), the particle removal efficiency of the first lot was 72%, while the particle removal efficiency of the fourth lot was 91%.
Generally, as described in JP10-109072A, the removal efficiency of particles is considered to be susceptible to the dissolved gas concentration of the cleaning liquid. Thus, in order to restrain a variation in the removal efficiency of particles between lots, it is effective to make uniform the dissolved gas concentration of the cleaning liquid.
However, an instrument for measuring a dissolved gas concentration which has a chemical resistance is considerably expensive. Thus, in consideration of a price of a general substrate cleaning apparatus, incorporation of such a dissolved-gas concentration measuring instrument having a chemical resistance into the substrate cleaning apparatus for a mass production is not practical. Therefore, under the existing circumstances, it is difficult to grasp a variation of the dissolved gas concentration of a cleaning liquid. In addition, a dissolver for dissolving a gas in a cleaning liquid, and a degasifier for deaerating a cleaning liquid require a large installation space, and complicate the control of the substrate cleaning apparatus. Namely, even if the dissolved gas concentration can be grasped, another problem arises.